This invention is related to the filters for eliminating D.C. power supply ripple and other A.C. signals in the D.C. power supply lines, and more specifically to filters having active elements as well as those filters which generate the necessary power for their elements
In an audio amplifier, different audio signals exist at different points within the amplifier circuit at any one point in time. If these separate audio signals are allowed to meet at some point in the circuit, they combine into a complex signal. This complex signal reaches the main signal path in the amplifier and is eventually heard as part of the actual output from the speakers, thereby resulting in an output signal which is not a true representation of the original input signal. This quality of signal reproduction is usually not satisfactory, especially for the perfectionist in high fidelity sound systems.
Fortunately, there are generally only two places in an amplifier where this mixing of the different audio signals occurs. The first mixing location is at the system ground and can be eliminated in a generally acceptable manner by providing a common grounding point on the amplifier chassis for the various individual circuit systems.
The second place within an amplifier where mixing can occur is the D.C. power supply line, or B+ voltage supply line. The usual technique for solving this problem is to use an electrolytic capacitor as a filter for the power supply ripple, which is around 120 Hz. Electrolytic capacitors are adequate for eliminating ripple frequency, but are not usually adequate for removing higher frequencies from the D.C. power supply lines because to build such a capacitor is virtually a physical impossibility
In addition, the D.C. power supply should be designed to provide a constant output voltage, even as the load of the amplifier circuit varies. A filter connected to the D.C. power supply output, which monitors the output voltage and automatically adjusts it to a predetermined level, can help provide a constant output voltage despite variances in load and input voltages.